1. Chief, Gastrointestinal Section, Division of Biology and Human Nutrition, Institute of Nutrition of Central America and Panama (INCAP), Carretera Roosevelt, Zone 11, Guatemala City, Guatemala, C.A.
2. Head, Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514.
3. Doctoral fellow, School of Public Health, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514.